There are a lot of questions about specific actions in the Endgame and how they affect the flow of time. There also many popular answers which are contradicting each other, therefore I'd like to ask here explicitly about the final result of timelines

Controversial Points:

I understood that every time someone is time travelling a new timeline is created. A lot of people are claiming that the Ancient One says everything would be happen the same way again, as long as the stones are returned.

Some claim that Thanos in 2023 would become a time remnant, but that's only known to exist in the DC franchise - not in the MCU.

So how many timelines do exist after endgame? Is it really only one merging all the alternative ones? (Only counting the ones which are mentioned or shown in the MCU movies.)

@Paulie_D That's one of the posts which triggered my question, because I think it is not as easy as just putting the stones back. They didn't teleport them out, therefore they can not bring them back without anybody noticing. Also it's only about one potential branch.
– ChrisApr 28 at 14:07

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@Paulie_D They knocked out Quill at the time we saw him in the beginning of Guardians of the Galaxy. How should they undo this?
– ChrisApr 28 at 14:16

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I don't know why this has attracted so many downvotes, it's a perfectly sensible question. One timeline they created has Loki escaping with the Tesseract after the battle for New York instead of being taken back to Asgard, so the answer is definitely not '1'.
– Crow T RobotApr 28 at 18:04

4 Answers
4

Joe Russo confirmed that it is not a closed timeloop. "You can't change the future by simply going back to past. But it's possible to create a different alternate future. It's not butterfly effect. Every decision you made in the past could potentially create a new timeline."

As there is an unlimited number of universes, we'll look only at timelines, that were actually affected in the movies. So Nebula will always open the tunnel for Thanos and that timeline will always lose their Thanos.

As Joe Russo explained Captain America's last journey, it's most likely that the main universe is the one which wasn't affected by time traveling at all.

Timelines

Dr. Strange checked 14000605 possible futures to select the one in which they are winning. It's unclear if these timelines were actually created at that moment, but at least the one they take is shown in the movie.

Hawkeye created a new timeline by taking the baseball glove from an unknown date in the past.

By knocking out Quill in 2012 War Machine created a different future, the events of Guardians of the Galaxy Vol. 1 won't happen like that again. Quill won't finish his dance and won't get the Power Stone.

Thor and Rocket went to Asgard in 2013 to get the Reality Stone and stole Mjölnir from the alternate Thor.

The bigger part of the Avengers in 2012 stole the Space Stone and Black Widow sacrificed herself so that they can get the Soul Stone.

Because they failed in 2012, Captain America and Iron Man had to go back to 1970 to get the Tesseract.

After returning the Infinity Stones, Captain America traveled to around 1940 to live with Peggy Carter. Joe Russo stated that this happened in an alternate reality and Cap probably didn't hide there. He had to make another jump back to the main universe, but we don't know how.

Additional Notes:

Thanos and his army are traveling from one of the alternative branches to 2023 in the main universe. We assume that their behavior is deterministic and they will always leave that universe in that branch.

Captain America didn't create any relevant timelines when returning the stones. As Joe Russo explained there is a unlimited number of universe splitting based on the decisions made by it's inhabitants, but they are not relevant for this question.

If every single change creates a new timeline then we wouldn't have seen Old Man Steve in our main timeline. He'd have branched off into another one. This suggests it takes a pretty big splash to change the river's course. And once you take that into account, many of the items on your list can be taken off.
– RikaelusMay 1 at 4:15

I hadn't even thought about the Tesseract being destroyed, that's a massive change. The aether was also a stone afterward, injected that into poor Jane must have chafed.
– GordonBennettMay 1 at 8:12

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@Chris Except, according to the Ancient One's description, taking the stones and returning them would maintain the timeline. That means that time travel, alone, isn't enough to create new timelines. Combine what everyone says and only substantial changes will cause completely new timelines to form. Steve went back in time at the end but kept a low enough profile that a new timeline didn't form, allowing us to see him as an old man in the "prime" timeline all the other characters were still in.
– RikaelusMay 1 at 9:26

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@Rikaelus The Ancient One's explanation was very ambiguous in my opinion. It's unclear if she was talking about the stones only or time travel in general. I assume we have to wait for interview statements by the producers to get clarity. In the mean time, I chose the variant which makes most sense to myself.
– ChrisMay 1 at 12:43

tl;dr five or three timelines, depending on if time travel always creates a new timeline or not

Assuming Banner's both statements about time travel are correct:

The grandfather paradox cannot happen, so time travel always creates a new timeline since some things are bound to change (the butterfly effect, but the changes can be minor). The very act of time travel introduces a change.

Returning the infinity stones "uncreates" those new timelines. The Ancient One gives the Time stone to Banner trusting the stones will be returned, suggesting Banner is correct as one would assume the keeper of the Time stone know this time stuff. The "uncreating" cannot mean that those timelines would just cease to exist, as Captain America who is returning the stones needs to exit those timelines. Therefore, these timelines are merged with the original timeline (the taking and returning of the stones happens during the same timeline; with one exception).

As mostly already pointed out by @Chris (see the graph), we have the following timelines (pre-merging of timelines due to the return of infinity stones):

Original MCU timeline (where Endgame starts)

Trial time travel, Hawkeye takes a baseball glove

Time travel to Asgard, Reality stone and Mjolnir taken, present-Thor talks with his mother

Time travel to Morag, Power and Soul stone taken, Star-Lord is knocked out, Thanos from this timeline travels to the original MCU timeline and is killed

Cap returns the stones to Morag and Vormir (5,6->1; Gamora from timeline 5 presumably still alive in timeline 1)

Time travel to 2012, Mind and Time stone taken, Loki gets hold of the Tesseract and disappears, future-Cap meets past-Cap and tells him that Bucky is alive, past-Ancient One talks with future-Banner

Cap returns Mind and Time stone to 2012 (7->8; Loki still has escaped so this timeline cannot merge to timeline 1; it is unclear if Cap even needs to return the stones to this timeline)

Time travel to 1970 taking Space stone (Tesseract), future-Tony talks with his father

Cap returns the stone to 1970 (9,10->1)

Cap travels (presumably) to 1940 and lives with Peggy Carter

Old Cap time travels to the "original" timeline, effectively creating a new timeline (Endgame ends here)

It is unclear how Cap restores the stones to their original form (space stone as Tesseract, Mind stone in the scepter, Aether in Jane).

The merging of timelines implies Tony always talked with his father and Thor with his mother and Star-Lord was always knocked out. Also, the infinity stones were always taken and returned in the original MCU timeline, except for the Mind and Time stones, and they were absent for a moment since Cap cannot return them at the exact moment they were taken lest someone spots him. However, in the original timeline Cap never comes back from his quest of returning the infinity stones, as he needs to time travel to get there and thus he creates a new timeline (since he does not return through the time machine). The official MCU timeline at the end of the movie is therefore a new timeline different from the one Endgame started with, because, for whatever reason, Cap chooses to time travel to that moment instead of returning through the time machine.

We have a total of 5(+) separate timelines:

Original MCU timeline where Cap never returns

New MCU timeline where old Cap returns

Timeline where Loki escapes with the Tesseract, the future and past Caps meet and the Ancient One talks with Banner

Timeline where Cap marries Peggy, otherwise similar to original MCU timeline

Timeline where Hawkeye took the baseball glove, otherwise similar to original MCU timeline

Note that the latter two timelines are presumably otherwise similar to the original MCU timeline: the snap happens, Avengers travel in time and create the above mentioned timelines again, unless the butterfly effect changes things up.

As if this wasn't long enough already...

Alternatively, we could assume that time travel does not necessarily create a new timeline and minor changes can happen. Major changes still create a diverging timeline. Minor changes introduced by time travel like the missing baseball glove or Cap laying low in the background from 1940 to present could still happen on the original timeline (meaning these always happened).

There are then three timelines in total:

Original MCU timeline (including two Caps at the same time, one being married to Peggy)

Timeline where Loki escapes with the Tesseract in 2012

Timeline where Thanos and his army are gone (traveled to the future and got snapped), Gamora is missing (presumably in 2023 in original MCU timeline), perhaps also the timeline where Star-Lord was knocked out (thanks for @Rikaelus for pointing this timeline out, see their answer)

I do not remember the timing of removing the infinity stones and Loki getting the Tesseract in 2012 or if it was made clear in the movie. It could be possible for Cap to choose the points of time when he returns the Mind and Time stones so that the timeline of Loki escaping is somehow isolated. This leads to Banner always talking to the Ancient One and Cap always meeting his future self in 2012. Future-Cap could have erased past-Cap's memory of the exchange with the scepter(?). He probably did the same to the Hydra people.

However, this form of time travel has more ambiguous rules. If these things always happened in the past, it means that the time travelers need to introduce these changes exactly how they happened. Perhaps their actions and the flow of time is somehow regulated by the infinity stones. How can they then fail to produce the past exactly as it was and create a new timeline?

Personally, I prefer the three timelines outcome, and it seems to be the one hinted by the movies, despite the more ambiguous time travel rules. Peggy Carter is known to be married but the husband's identity is not revealed, hinting it might have been Steve all along.

If Cap'n America is aware of the missing glove (and I don't know how he wouldn't be), he could return that to its original spot as well.
– Jesse AmanoApr 30 at 1:54

Good point about Captain' last journey, I was struggling with that.
– ChrisApr 30 at 5:19

In your first part you list some moments that are contradicting your second part about "everything happened before". If there is not a lot of hand waiving that cannot work.
– ChrisApr 30 at 5:21

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I agree with your alternative logic, but I'd posit an additional timeline for a total of three. I'll spare the long form here since I provided the explanation in another answer. I would add that I think he actually did change his timeline's history in regards to Peggy, but it wasn't substantial enough that a new timeline was formed. The river kept its course despite the ripples Cap caused.
– RikaelusMay 1 at 4:18

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@Rikaelus I like the river logic you present. It makes the most sense and it seems to be the explanation the movie intends (e.g. old Cap returning), even though it goes against Banner's statement that you can't change the past. Perhaps he just wasn't fully informed at that point. I think you are right, if the timeline where Loki escapes is not "uncreated" by the returning of the infinity stones, then the one where Thanos travels into the future and is snapped should also still exist. I'll edit my answer accordingly.
– GalacticMMay 1 at 13:40

I've taken the Ancient One's explanation to mean that there's a little bit of the "river" element to timelines. So long as you don't redirect the river too far, it'll eventually return to its original path. So long as the Infinity Stones are returned to their respective times, the river won't branch into a completely different direction.

I think there were two key situations that caused separate timelines to branch out.

Loki taking off with the Tesseract and how Steve took the Mind Stone. Loki strutting around the universe with the Tesseract and the Hydra agents thinking Steve is one of them would cause huge causal ripples that would certainly preclude a temporal reconciliation. Sorry, Ancient One, but the timeline you'll know? You're in for some crazy.

Thanos, Gamora, and Nebula jumping forward in time. Their sudden absence "in the past" would obviously cause a huge change in the series of events we know. Gamora would never get in her 3-way fight with Quill, Rocket, and Groot. The Guardians wouldn't form. The Collector might end up with the Power Stone. Ronan probably goes boring space pirate. Xandar is safe. The snap never happens. Stark, Black Widow, and Vision don't die. Ego would still be out there waiting for a viable offspring to return. You get the point.

So ultimately we have our main timeline, plus those two. So three total.

Addendum #1: This can also be supported by the fact that Steve went back in time, ostensibly changed Carter's life quite a bit, and yet he showed up as an old man in the main timeline. So even though he changed history, it wasn't enough to create a distinct timeline. Obviously, he had to have kept a pretty low profile the whole time, too.

Addendum #2: The "river" aspect of time travel is coincidentally what Hank McCoy theorizes in X-Men:DoFP for that universe

I would be really interested in a more fleshed-out narrative of the events of timeline #2 after Thanos, Gamora, and Nebula "leave" that timeline. Would be a fun thought exercise to extrapolate everything that would have been different.
– kelnosMay 29 at 19:59

@kelnos Agreed. I think Disney was planning a "what-if" animated show based on MCU events, so maybe we'll get an episode for that.
– RikaelusJun 20 at 19:15

To keep things simple I think their are 2 timelines : One where Thanos snapped and one where Thanos is dead (MCU activities don't happen).

Timeline 1 : Thanos snapped.
Let's get a revision.
In 2012 Loki has the scepter from Thanos and whole tesseract thing happens. Then MCU activities take place. Thanos collects the stones in 2018. Kills Gamora. Then snaps and erases 50% population. Later Thor kills Thanos(2018 one). 5 years later remaining avengers time travel. Go back to the past, collects the infinity stones. Come to present. Reverses the snap. Places the stones back. Done.

But remember past Nebula is killed and old Thanos (one which time travelled) is dead (in endgame after Ironman snaps). So because the old Thanos is dead there's no one to give Loki the orders to bring the tesseract. MCU activities don't happen. Gamora isn't sacrificed. No one snaps and erases 50% population in 2018 (because there's no Thanos). So basically in this new timeline we don't get to see activities related to Thanos.

Regarding Captain America's decision to live in the past we can say that when he went back to Peggy there are 2 Captains present (all this time). One in water and one with Peggy. Maybe his husband and children which we don't know about, are of our own old Captain America. He confronts Peggy that he's from the future and he came back for her. Never throughout the MCU we are told that Peggy knows nothing about future.
He lives his life in hiding with Peggy. Tells her the truth that in 2012 his current self will come out from the water.

Regarding Nebula .... The one who time travels with Thanos and our Nebula of GOG, can be considered as two separate machines (humanoids). A soul is irreplaceable but a machine (parts) can be replaced. So why is Nebula alive can be justified by saying that both Nebula's have a difference of 10 years (one from 2012/13 and one from 2023) and within this 10 years she has replaced each and every part of her machine-like body.

I know this might not be satisfactory and difficult to understand but this is the best possible explanation which justifies EVERYTHING with minimum confusion.